The global positioning system (GPS) is a satellite-based radio-navigation system built and operated by the United States Department of Defense. The system uses twenty-four satellites orbiting the earth at an altitude of about 11,000 miles with a period of about twelve hours. Some additional satellites may be present as spares. These satellites are placed in six different orbits such that at any time a minimum of six satellites are visible at any location on the surface of the earth except in the polar region. Each satellite transmits a time and position signal referenced to an atomic clock. A typical GPS receiver locks onto this signal and extracts the data contained in it. Using signals from a sufficient number of satellites, a GPS receiver can calculate its position, velocity, altitude, and time.
A GPS receiver can operate in many modes. In a “hot start” mode, the receiver already has the time, its last position, and the information on satellite position (also known in the art as almanacs or ephemeris) stored in its memory. The receiver can use this stored information to determine which satellites are probably visible, and it can then lock onto those satellite signals in a short time. On the other hand, the receiver may have no prior data on its position, time, or almanacs stored in memory. In this “cold start” mode, the receiver has to search for signals from all of the satellites present in the constellation. There are some other modes where partial information on time, position and almanacs are available and the corresponding start mode is known as “warm start.”
The GPS receiver has to acquire and lock onto at least four satellites in order to derive the position, velocity and time. Usually, a GPS receiver has many parallel channels, each receiving signals from a separate visible GPS satellite. The acquisition of the satellite signals involves a two-dimensional search of frequency and the PN code phase. Each satellite transmits a unique PN code which repeats every millisecond. The receiver locally generates a replica frequency and a replica code phase and correlates these with the received satellite signals. The PN code has to be searched in at least 2046 phases and the frequency search depends upon the Doppler frequency due to relative motion between the satellite and the receiver. Additional frequency variation may result due to local oscillator instability.
The GPS receiver computes an estimate of the line-of-sight distance from the satellite to the receiver which may include errors due to receiver clock bias, and other effects. This estimated distance is known as the pseudo-range. The estimate of the pseudo-range often contains additional errors due to multi-path, i.e., the reflections of the signals by many objects such as buildings, mountains, etc., as the signals propagate from the satellite to the receiver. This reception of both line-of-sight and reflected signals often results in the computation of inaccurate pseudo-range, and thus also introduces errors in the estimated position of the receiver. Due to the superposition of the direct and reflected signals (which are slightly delayed), the resulting correlation pattern deviates from its usual triangular shape exhibiting a multi-peak correlation curve. The earliest correlation peak corresponds to the direct signal, but the position of the peak may be shifted from its true position due to the superposition of the direct and reflected signals. This shift in the position of the correlation peak results in pseudo-range error and error in the computed receiver position. Further, the early and late correlators adjust their values to be equal and force the prompt correlator to remain at the center. Thus the prompt correlator represents the wrong peak. This will be discussed later in this section. This reception scenario may be more complex in the presence of a plurality of reflected signals.
There are several known methods available to estimate and compensate for the error due to multi-path. One of the most popular and widely used method is the use of narrow correlators as explained in the paper titled “NovAtel's GPS receiver—The high performance OEM Sensor of the Future” by P. Fenton et al presented at ION GPS-91, September 1991. Additional details on narrow correlator spacing may be found in the paper, “Theory and Performance of Narrow Correlator Spacing in a GPS Receiver” by A. J. Van Dierendonck et. al., Journal of The Institute of Navigation, vol. 39 no. 3, Fall 1992. Another paper “A practical Approach to the reduction of Pseudorange Multipath errors in L1 GPS receiver” by B. R. Townsend et. al. presented in ION GPS-94, Sep. 20-23, 1994 illustrates a method of multipath mitigation but this method requires a lot of computational power. Further a Delay Lock Loop approach also known as MEDLL has been describe in, “The Multipath Estimating Delay Lock Loop Approaching Accuracy Limits” presented at the IEEE Position, Location, and Navigation Symposium on April 1994 by R. Van Nee et. al. and also requires more computational power. A multipath mitigation approach using modernized GPS Signals has been discussed by L. Weill in his paper titled, “Multipath Mitigation Using Modernized GPS Signals: How good Can it Get?” presented at ION GPS 2002, September 2002 with pp. 24-27. This approach can be implemented only upon the modernized signals that are available in the GPS system. The correlators in a spread spectrum or GPS receiver give the correlation values for different phase shifts between the received and local PN sequence. Usually the separation is about half of a chip, where a chip is one bit of the PN sequence. In GPS navigation, a half-chip delay corresponds to about 150 meters of distance. Therefore, if the reflected signal has a pseudo-range which is 150 meters more than the direct signal, it contributes to the energy of the next correlator also known as the Late (L) correlator, rather than to the correct correlator known as the Prompt (P) correlator. Thus, the resulting correlation curve may shift the peak towards the L correlator. When the pseudo-range change is different from 150 meters, the correlation curve may take a different shape. When the correlators are placed closer than a half chip apart, a reflected signal may peak in one of these correlators, so that the direct and reflected signals may be separated. The use of narrow correlators, however, increases the hardware complexity and power consumption. Use of a multi-antenna system to nullify the gain in the direction of multi-path is another technique employed which is useful mostly in static conditions. The ground-plane and helical shield are useful only under static conditions. Other methods of multi-path mitigation include one based on the level of stability of the pseudo-range as given in U.S. Pat. No. 6,484,098, one based on the use of L1 and L2 signals as given in U.S. Pat. No. 5,185,610, one based on the use of multi-bit correlators as given in U.S. Pat. No. 6,393,046, one based on data bits as given in U.S. Pat. No. 5,963,601, a velocity based method as given in U.S. Pat. No. 5,771,456, a satellite trajectory based method as given in U.S. Pat. No. 5,726,659, may be based on the variation in SNR, using wavelets or Maximum Likelihood (ML) or Minimum Mean-Square-Error (MMSE) methods. However, most of these methods may not provide good multi-path mitigation below a certain value of multi-path length or in some other cases may involve a lot of computation. Some of these techniques are useful only when one multi-path component is present while in practice there may be many possible reflected components.
Recently, multi-path mitigation techniques based on correlator outputs such as the Early (E), Prompt (P), Late (L) have been developed. These are based on the fact that the earliest component at the receiver is the direct signal while various reflected signals arrive later and contribute to the later correlator outputs. Published U.S. patent application 2005/0032477A1 from Qualcomm Inc. uses stored correlation curves and compares them with the present correlation curve to determine the multi-path. This technique also includes mathematical models. However, this technique requires lot of storage memory and also the comparison may not hold good under all conditions. U.S. Pat. No. 5,692,008 assigned to NovAtel and U.S. Pat. No. 6,917,644 assigned to SiRF determine the shift in Prompt (P) correlator position due to the requirement E=L, the E and L being subjected to different levels of reflected signal power. The SiRF patent takes into account only the E, P and L correlators in determining multi-path while the NovAtel patent does not specifically give any number of correlators. Published U.S. patent application US2004/0208236 A1 from NovAtel discloses the use of correlation curve shape or Pulse Aperture Correlator (PAC) in determining the multi-path. However, this approach requires a lot of computation and associated hardware. Another method for mitigation of the multi-path effect is the double delta technique. This technique uses five correlators but has the disadvantage that it requires a high precision measurement of E2, E1, L1 and L2 values because the measured differences (E1−L1) and (E2−L2) are usually very small.
Most of the above multipath methods require a set of narrowly spaced correlators. However, providing a large number of correlators results in increased gate count in the hardware, which increases the power and the physical size. Further, it is not possible using these methods to adaptively change the correlator spacing as may be required in varying multipath conditions.
Accordingly, there is a need in the art for a navigational satellite signal receiver to be able to detect and compensate for the multipath effect without increasing the number of correlators or power consumed. There is also a need for a receiver that can adaptively change the correlator spacing for varying multipath conditions.